This invention generally relates to a system and method for automatically transferring a cryogen, such as liquid carbon dioxide, from a higher pressure storage pressure tank to a lower pressure transportable tank without the need for continuous, manual adjustments of flow control valves.
Air conditioning and refrigeration systems of the type used to cool the loads of trucks and tractor-trailers conventionally utilize a chloroflurocarbon (CFC) refrigerant and a mechanical refrigeration cycle. Because of the suspected depleting affects of CFCs of stratospheric ozone (O.sub.3), practical alternatives to the use of CFCs in such refrigeration systems are being sought. One such alternative is a cryogenic refrigeration system utilizing either liquid carbon dioxide or liquid nitrogen. Such a system is particularly attractive because, in addition to eliminating the need for CFC refrigerants, it also eliminates the need for a refrigerant compressor and the diesel engine or other prime mover that drives it. An example of such a cryogenic refrigeration system that is designed for use with liquid carbon dioxide is described and claimed in U.S. patent application Ser. No. 08/501,372, filed Jul. 12, 1995, and assigned to the Thermo King Corporation.
When such cryogenic refrigeration systems are used to cool the contents of a truck or tractor-hauled trailer, they are powered by means of a transportable storage tank that is small enough to be easily hauled by the vehicle, yet large enough to contain enough liquid cryogen to keep the contents of the truck or trailer cool for a practical length of time. Ideally, the liquid carbon dioxide within the transportable tank is maintained at a pressure of approximately 110 psi, which keeps the cryogen at a temperature of approximately -50.degree. F. Liquid carbon dioxide maintained at these conditions is well suited for transportable refrigeration applications as it has a relatively large heat absorption capability in combination with a high density. However, liquid carbon dioxide that is manufactured and stored in large storage tanks is maintained at higher pressures on the order to 250 psi to 300 psi, and higher temperatures on the order of 0.degree. F. to -10.degree. F. Liquid carbon dioxide maintained at these conditions has a relatively smaller heat absorption capability and lower density. Hence if the liquid carbon dioxide is to be optimally used, it must undergo a substantial pressure drop (300 psi versus 110 psi) when it is loaded from a storage tank to the tank of a transportable cryogenic refrigeration system.
In the prior art, in order to fill the tank of a transportable cryogenic system from a storage tank, it was necessary for the system operator to install an inlet conduit between the storage tank and the transportable tank, and a vent conduit to a vent opening in the transportable tank. Flow valves and pressure sensors were provided in both the inlet conduit and the vent conduit. During a filling operation, it was necessary for the system operator to continuously manipulate the fill valve and the vent valve while observing pressure gauges in order to fill the transportable tank from the storage tank at a pressure of approximately 110 psi to optimize the cryogenic properties of the liquid carbon dioxide.
However, the applicants have observed a number of risks and shortcomings associated with such a tank filling technique. For example, it is important that the pressure of the liquid or liquid/vapor mixture in the system not fall below the triple point for CO.sub.2. At or near this point, a solid-liquid mixture (slush) begins to form and this can cause blockage of the lines. Solid CO.sub.2 is generally referred to as dry ice. Dry ice in the fill hose can block the flow of liquid carbon dioxide, while dry ice in the transportable tank renders the cryogenic system inoperative until the carbon dioxide becomes reliquified. The reliquification process is always tedious and lengthy, and can take as long as several days if the inner vessel of the transportable tank is made of mild steel rather than stainless steel, since the -110.degree. F. temperature of dry ice can embrittle mild steel to a point where it can rupture if the transportable tank is suddenly repressurized. The filling of the transportable tank at too high a pressure can blow out safety devices such as over-pressure disks mounted in the walls of the vehicle tank.
Because of the necessity of maintaining a proper pressure range, prior art filling systems require a trained and experienced system operator to continuously manipulate both the fill and vent valves during a filling operation while monitoring the pressure of the transportable tank. Such an operator must also be able to accurately estimate when the transportable tank is full, since the overfilling of such a tank can also cause unwanted dry ice formation in both the transportable tank and the inlet conduit. The operator must also manually purge the fill hose of liquid carbon dioxide after the filling operation is complete to avoid the formation of potentially obstructive dry ice. Finally, the applicants have observed that the continuous venting of gaseous carbon dioxide generated by the pressure differential between the storage tank and the transportable tank causes a continuous loud "screaming" sound that necessitates of ear protection of not only the operator, but of all persons in the immediate vicinity of the filling operation.
Clearly, what is needed is a system for transferring liquid carbon dioxide from a high pressure storage tank to a lower pressure transportable tank which can perform a filling operation automatically, thereby obviating the need for continuous valve manipulation and gauge monitoring by an experienced operator. Ideally, such a system would also include some sort of means for automatically shutting off the filling operation when the transportable tank attained a full condition so as to prevent overfilling and the unwanted formation of dry ice in the tank and filling hose. It would also be desirable if such a system includes some sort of means for automatically purging the filling hose at the termination of a filling operation in order to prevent not only the unnecessary wastage of liquid cryogen, but the unwanted formation of dry ice in this hose. Finally, such a system should have some sort of means for reducing the noise generated by the continuous venting of gaseous carbon dioxide so as to obviate the need for ear protection in the vicinity of the filling operation.
The foregoing illustrates limitations known to exist in present systems for transferring liquid carbon dioxide from a high pressure storage tank to a lower pressure transportable tank. Thus it would be advantageous to provide an alternative directed to overcoming one or more of those limitations. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.